High energy gamma rays from complex particle collisions

High energy γ-rays (E/sub γ/ approx. 10 to 30 MeV), from the decay of Gaint Dipole Resonances built on excited nuclear states (e.s.-GDRs) are a common feature of all energetic nuclear collisions. Current results indicate that for bombarding energies below 5 to 6 MeV per nucleon the statistical emission of high energy γ-rays predominates in complex particle collisions. Statistical GDR properties have now been studied over a wide range of mass, energy and angular momentum. Recent results include: (1) in light compound nuclei considerably broadened e.s.-GDRs are observed, with strengths that indicate relatively pure compound nuclei isospin, (2) in medium mass nuclei (A approx. 60 to 80) e.s.-GDRs are found with substantially reduced strength (approx. <0.5 of the classical dipole sum rule) and (3) in rare-earth deformed nuclei a splitting of e.s.-GDRs is apparent, indicating the persistence of deformation at elevated temperature. At higher bombarding energies (6 to 9 MeV per nucleon) for 3He and 4He strong enhancements are seen for E/sub γ/ approx. > 15 MeV, due to nonstatistical effects. These enhancements are associated with strong forward-backward asymmetries, arising from (phase coherent) E1-E2 interference, which peaks at energies near the isovector e.s.-GQR. 26 references